A lithographic apparatus is a machine that applies a desired pattern onto a target portion of a substrate. Lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that circumstance, a patterning device, such as a mask, may be used to generate a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target portion (e.g. including part of, one or several dies) on a substrate (e.g. a silicon wafer) that has a layer of radiation-sensitive material (resist). In general, a single substrate will contain a network of adjacent target portions that are successively exposed. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion in one go, and so-called scanners, in which each target portion is irradiated by scanning the pattern through the projection beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction.
A lithographic apparatus includes components, such as radiation sources, that generate heat. Other elements in the lithographic apparatus, such as support frames and optical and other elements are subject to a thermal load from the heat generating components. It has been found that the thermal loads experienced by certain elements in the lithographic apparatus compromise the imaging quality of the apparatus. It is an aspect of the present invention to address this problem.
Further, to reduce the size of features that can be imaged using a lithographic apparatus, it is desirable to reduce the wavelength of the illumination radiation. Ultraviolet wavelengths of less than 180 nm are therefore currently used, for example, 157 nm or 126 nm. Also used are extreme ultraviolet (EUV), also referred to as soft x-rays, wavelengths of less than 50 nm, for example, 13.5 nm.
A consequence of using shorter wavelengths is that while thermal stability within the projection system continues to be a problem in lithographic apparatus, it has been found that apparatus operating at shorter wavelengths, such as those less than 180 nm, are even more prone to thermal stability problems than those operating at higher wavelengths.
An apparatus operating at a shorter wavelength is described, for example, in co-owned pending European Patent Application No. 1178357. In EP1178357, a lithographic apparatus is known of which certain components are located in a vacuum chamber. The projection beam images the mask onto the substrate via a number of mirrors. Such an arrangement is needed, for example, when an EUV beam is used, because an EUV projection beam would be unsuitable for projection purposes in gases at atmospheric pressure, and because no refractive optical elements are presently available for EUV radiation. The same arrangements are necessary for other types of beams.
European Patent Application No. 1178357 notes that operation under vacuum may cause temperature stability problems, because heat radiation from the walls of the vacuum chamber or from the vacuum pump may lead to thermal expansion or contraction. This results in imaging errors when temperature critical components like the reference frame, the support frame, the substrate table, or the projection system experience temperature variations.
In particular, thermal stability affects the imaging quality because the position of the optical elements, in particular, the mirrors in EUV lithography projection apparatus, needs to be achieved and maintained during the exposure extremely accurately. For example, in EUV apparatus, the mirrors need to be positioned with an accuracy of +/−0.1 nm or less. Since the optical elements, such as mirrors, are supported on a support frame, it will be understood that temperature variations which cause the support frame to deform may also cause the position of the mirrors to vary.
Conventionally, this problem has been addressed by designing apparatus so that they react as little as possible to thermal variations. For example, by constructing support frames and other critical structures of materials such as ZERODUR® or INVAR®, which are specialty materials referred to in the art as NZTE (near-zero thermal expansion) materials, or the like. Such materials are designed to have very low expansion coefficients. Although the use of these materials improves the thermomechanical stability of the apparatus, they have various drawbacks including cost, manufacturability, and fragility of the materials used, especially ZERODUR®, add to the complexity of the manufacture. Also, connecting glass materials together is problematic, which is one of the factors contributing to the more general problem of manufacturability of NZTE materials, such as ZERODUR®.
A further approach to solving the problem of thermal stability, as for example, described in EP1178357, is to provide a heat shield disposed between heat sources and temperature critical components.
It has been found that in spite of efforts to minimize the effect temperature fluctuations have on the lithographic apparatus operating at any wavelength, this impact cannot be fully eliminated. Further as mentioned above, the materials and techniques used to combat the problem of thermal stability, have their drawbacks that generally add to the complexity and to the cost of the apparatus.